The major objective of well logging is to obtain quantitative and qualitative information related to hydrocarbons in earth formations surrounding a well. In many nuclear well logging techniques the formation and borehole are irradiated by energetic nuclear particles such as neutrons and gamma rays. These particles may either be scattered in the formation or the borehole or they may undergo a nuclear reaction which results in the creation of a new particle or gamma ray. The scattered particles or newly created particles can be detected at one or several locations and by one or more detectors. The number of energy particles detected and their energy levels generate signals that contain information about the irradiated earth formation and the borehole. During the logging process, various processing techniques can be applied to these signals in order to get the desired logging information from the signal. One of these processing techniques is comparing features of these signals with other known data to make certain determinations about the information contained in the signals.
Many of the nuclear well logging techniques compare the measured signal with additional information. Many of the techniques compare the measured signals with other data based on spectral analysis of the measured signal. As stated earlier, these signals are sometimes gamma rays that result from interactions of atoms with neutrons emitted from a downhole radiation source. Such gamma ray signals are representative of certain atoms of the lithology of the formation (i.e. the matrix or the formation fluid) or of the borehole. In one example of spectral analysis, the energy spectrum of gamma rays resulting from the capture of the neutrons is decomposed into contributions due to individual formation elements, usually called "elemental yields". These elemental yields reveal information concerning the presence of earth formation elements such as hydrogen, silicon, calcium, chlorine, sulfur and iron. Important petrophysical parameters such as porosity, matrix lithology and water salinity may be derived from the elemental yields. An example of a capture gamma ray spectra analysis is depicted in U. S. Pat. No. 3,521,064 to Moran et al.
In Moran, a measured gamma ray energy spectrum, representative of a formation of unknown composition, is compared with a composite spectrum constructed from individual laboratory derived standard spectra of the constituents postulated to comprise the formation. This standard spectrum is composed of standard responses from certain formation elements. The different element responses of the standard spectra (elemental yields) which give the best fit to the measured spectrum when weighted by each element sensitivity (i.e. the ability of an element to emit gamma rays and be detected) represent the relative proportion of the constituents of the formation. "Fit" can be thought of as the closeness of the match between corresponding points of the measured and standard spectra. (If the signal was superimposed over the other signal, how close would the signals match/fit). In other well logging techniques, the calibration of the signal equipment is very important to the measurement of the signal. In these calibration processes, reference signals are used to align the measured signal with certain time or energy parameters. If there is not an adequate alignment between the reference point and the appropriate feature of the signal, an adjustment of the signal amplitude may be necessary to align the signal with the calibration reference. In addition, an adjustment of the signal amplitude may be needed to provide an adequate signal for measurement.
Since the alignment of features from the measured spectrum with features of a standard spectrum may be crucial in these techniques, it is important to guard against drifts and other variances between the measured spectrum and the standard spectrum. Any drifts that occur along the signal chain during the course of the measurement can cause peaks in the measured spectrum to broaden or become otherwise distorted. If this occurs, the proper fit may not be obtained and measured information could be lost. These drifts can develop through changes in temperature of the detector or associated electronics, gradual changes in voltage levels, or variations of the gain of the various active elements in the signal chain. Despite the best efforts to control temperature and other environmental conditions, spectra taken over long periods of time with high resolution detectors often suffer an apparent loss of resolution due to these drifts. In some detectors, large changes in counting rates can also lead to apparent gain changes over fairly short periods of time.
One parameter that is important in controlling measurement drifts and adjusting signal amplitudes is gain. The gain is the change in the signal power or amplitude necessary to keep the signal at an appropriate amplitude level. In gamma ray or particle energy spectroscopy it is important that the gain of the system be known and constant at all times. If the gain is unknown it is difficult or nearly impossible to analyze the spectrum for its components. Small variations in the gain during the spectrum measurements will lead to a degradation of the spectrum resolution. If the gain variation is large the spectrum can no longer be analyzed and the information is lost.
During the detection of gamma rays, gain regulation for energy dispersive gamma-ray detectors in general and for scintillation detectors in particular has been done in many different ways. If the environmental conditions of a system, including the gamma-ray flux, can be kept constant it may be sufficient to do periodic calibration of the detectors at intervals of hours or days using radioactive sources. This requires that signal drifts due to temperature or equipment component changes be almost imperceptible.
One example of the importance of gain regulation is seen using scintillation detectors and photomultipliers (PMT). Many gamma ray detection systems use scintillation detectors and photomultipliers. The gain of a PMT can change due to small changes in the surface conditions of the electrodes in the interior of the PMT. These changes can occur as a consequence of tube start up or during long operation due to the constant electron (and ion) bombardment of the electrodes which are responsible for the amplification process in the PMT. The properties of scintillators do not change rapidly unless a catastrophic failure (breakage or chemical change) occurs. The light emission however depends strongly on the temperature of the crystal. If the temperature can be kept constant it is possible to stabilize the gain of the PMT by the use of a highly accurate light pulser which sends known constant amounts of light to the PMT. The gain of the PMT can be adjusted so that the light pulse always generates the same amplitude. This adjustment can be achieved by analog or digital techniques. However, this method does not correct for any changes in the scintillation light output.
In addition, in well logging the temperature of the tool in the wellbore is known to change dramatically from the surface (about 25.degree. C.) to the bottom of the hole where the temperature can reach 150.degree. C. and more. Under these circumstances the gain of a scintillation detector and its associated PMT can vary by a factor of two or more, therefore constant adjustment of the gain is necessary. Many applications require that the gain be kept constant to much better than one percent. Even if the detector is kept in a dewar the temperature changes are usually large enough to generate intolerable gain changes.
The main method of gain stabilization in this situation consists of using a radioactive source, the characteristic gamma-ray ray of which is used as a reference signal to stabilize the gain. This has the advantage that the gain of the entire system, including the preamplifier and ADC can be kept constant. However, in many applications the extra signal from the stabilization source is adding significantly to the background and therefore impeding the measurement of interest. This can be alleviated by using a gamma-ray source with a gamma-energy which is outside of the range of interest and/or by using various coincidence techniques. In the case of a neutron induced gamma-spectrum, the count rates in the gamma-spectrum are very high. At the present, this necessitates the use of a strong gamma-source for gain stabilization. This source however can be detrimental to parts of the measurements for which the count rate of interest is much lower. For example, the precision of the measurement of the formation capture cross section is strongly affected by a large number of background counts.
Although current methods of gain stabilization exist, there remains a need for a method which in the presence of a sufficiently high gamma-ray flux allows stabilization without the use of a strong calibration source or with no calibration source at all.